1. Field of the Invention
The present invention relates to a system for positioning a cradle for lowering and lifting a marine vessel relative to the height of the surface of a water body, especially when the height of the water body fluctuates over time, as with tidal action, and discounts the height of the water body as it relates to wave action.
2. Background of the Invention
Boat lifts and davits are well known to those who are skilled in the art. Parkins et al, U.S. Pat. No. 5,769,568, describes one of the more common layouts for a lifting cradle used to lift a marine vessel. When lifting or lowering a marine vessel, the operator typically must depress a control button, either remotely or while at the motor controls, and keep this button depressed until the proper level of the cradle has been achieved.
Recently those skilled in the art have added limit switches, or other sensors, to the motors to terminate the motors when the proper, fixed position has been achieved. However, the operator must still keep the control button depressed until the final desired position is achieved.
In regions where periodic and continuous changes in the water surface level is common, such as by tides or in rivers, these pre-defined and fixed positions will not allow the marine vessel to launch or to be retrieved unless precise conditions are met.
Endres et al, U.S. Pat. No. 5,593,247, describe a boat lift control system in which a plurality of elevations may be preprogrammed into the system corresponding to, for example, low tide, high tide, etc. The system is activated by a button, designated as ‘up’\or‘down’, and moves the lifting cradle to the next higher, or lower, position than the starting point, respectively, which may not be the desired position. The system is activated again and the lifting cradle continues on to the next preprogrammed position. This process continues until the desired position of the lifting cradle is achieved.
However, these preprogrammed states are not always useful. A typical situation would involve an outgoing tide, which results in a water surface level between two of the preprogrammed states. Use of the preprogrammed states would result in the lift either being too high or excessively low for efficient loading, launching or retrieving of the vessel.
Water level sensors are well known to those skilled in the art. Most often a water level sensor is developed and used to simply report when a given depth of water has been achieved and signals this condition to the user. Barrows, U.S. Pat. No. 5,515,025, and Jones, U.S. Pat. No. 3,995,251, describe water level sensors that determine the level of water relative to the boat trailer and then provide a signal to the user. However, these inventions are, admittedly, biased by wave action and changes to the angle of inclination of the trailer. Either of these situations could result in the marine vessel not seating properly and not providing the desired loading/launching/retrieving effect.